Welding has always a deteriorating effect on fatigue strength in structures under dynamic loading. Weld joints induce discontinuity and defects where potential fatigue cracks initiate. In the theory part of this thesis, the laser welding process is discussed in sense of fatigue.

The simulation possibilities for welding are introduced. With simulation, such effects as residual stresses, initial distortion and phase changes can be obtained. These residual states can be used in structure and tolerance design.

In this thesis, the suitability of fatigue assessment offered in rules and regulations is discussed. A large number of test results was collected from literature in order to determine the common fatigue assessment suitability for the fatigue design of laser-welded joints. Common assessment suitability for the fatigue design of laser-welded joint is discussed on the basics of the results.

In this thesis work, the effect of residual stresses on fatigue of welded joints was studied. In the study, the effect of residual stresses on the fatigue behaviour of the base material was studied. The results suggested that residual stresses play a significant role in fatigue. The statistical analysis of the probable defect size was done with test results from literature.